The present invention relates to lumped constant non-reciprocal circuit elements such as circulators and isolators for use in VHF, UHF and microwave frequency bands.
A circulator can be converted into an isolator if one port is closed with a dummy resistor, and an isolator can be converted into a circulator if one port is used as a connection port for external networks. Namely, they are of the same structure, though they are called differently depending on how they are used. Thus, lumped constant circulators and isolators have the same technological basis. Hence, it should be noted that though isolators are explained mainly hereinafter, substantially the same explanations are applicable to circulators.
A typical prior art isolator is shown in FIG. 8. The isolator has three sets of central conductors 401 positioned between two garnet plates 402, 402 which are insulated from the conductors by means of an insulator, and these elements are covered with a shielding plate 403. They are placed in the central bore of a ceramic substrate 404 with one end of each conductor 401 connected to a corresponding electrode [not shown]provided on the ceramic substrate 404. A permanent magnet 406 coated with an iron plate 405 is provided above the garnet plate 402. All of these elements are housed by the casing 407 and 408. Other elements such as hollow coils and capacitors may be implemented therein if required. Two ceramic substrates are often laminated.
Such prior art isolators have been made compact following the miniaturization of other microwave elements. For example, a very small isolator having an area of 20 mm.times.20 mm and a height of 10 mm has been realized, which uses garnet plates of 10 mm in outer diameter and 0.9 mm in thickness, and a permanent magnet of 13 mm in outer diameter and 4 mm in thickness.
Although prior art isolators have been made very compact, they are still much larger than other microwave elements. In particular, they are too high, protruding above surrounding elements.
In view of these circumstances, Nippon Ferrite previously filed a patent application for a lumped constant non-reciprocal circuit element comprising a set of mutually insulated central conductors, only a single garnet plate not more than 0.6 mm in thickness, a ceramic substrate having a central bore and a planar surface with patterned electrodes formed thereon, the set of mutually insulated central conductors being positioned within the central bore, and a permanent magnet positioned proximate the garnet plate for applying a dc magnetic field to the garnet plate, wherein each of the mutually insulated central conductors is folded to wrappingly enclose the garnet plate when received in the central bore of the ceramic substrate, and wherein the substrate is further provided with a plurality of terminals connected to the patterned electrodes to which terminal portions of the central conductor are operatively connected [Japanese Patent Laid-Open No. 63-107203 published May 12, 1988, corresponding to U.S. Pat. No. 4,812,787 issued March 14, 1989].
This lumped constant non-reciprocal circuit element will be described by an example of an isolator referring to FIGS. 9-12.
This lumped constant isolator comprises a circular shielding plate 501, three central conductors 502, a magnetic member 504, insulating sheets 505, an insulator substrate 506, three electrostatic capacitor-forming conductor layers 507, a conductor layer 508 formed on a bottom surface of the insulator substrate 506, a dummy resistor layer 509 and a ground electrode 510.
The three central conductors 502 extend radially from the circular shielding plate 501 and have terminals 503 at their tip ends. The magnetic member 504 is disposed on the shielding plate 501, and a first central conductor 502 is folded. After disposing an insulating sheet 505 thereon, a second central conductor 502 is folded and another insulating sheet 505 is disposed. Finally, a third central conductor 502 is folded to constitute a central conductor means. The insulator substrate 506 is provided with three electrostatic capacitor-forming conductor layers 507, a dummy resistor layer 509 and a ground electrode 510 for the dummy resistor layer 509 on a top surface thereof. The insulator substrate 506 is also provided with a conductor layer 508 over an entire bottom surface thereof. The ground electrode 510 on the top surface and the conductor layer 508 on the bottom surface are electrically connected to each other via a through-hole. The insulator substrate 506 has a central bore for receiving the central conductor means.
The insulator substrate 506 is disposed on a ground base 511, and the central conductor means is disposed in the central bore of the insulator substrate 506. Each of the terminals 503 of the central conductors 502 is connected to each electrostatic capacitor-forming conductor layer 507 formed on the insulator substrate 506 to constitute this lumped constant isolator. Incidentally, a magnet for applying a dc magnetic field to the central conductor means is disposed above the central conductor means.
In this lumped constant isolator, if the dummy resistor layer 509 and the ground electrode 510 are omitted, the isolator can be converted to a lumped constant circulator.
As described above, in each of these lumped constant isolators and lumped constant circulators, the central conductors and the shielding plate are formed integrally. Accordingly, the assembling is conducted by folding the central conductors to wrap the magnetic member. However, when the central conductors are folded, they tend to open due to their resilience, so that insulating sheets inserted between the central conductors are likely to be displaced. This might cause short-circuiting between the central conductors. In addition, the folding of the central conductors is sometimes troublesome. Also, if the central conductors are not folded precisely at predetermined angles, the desired characteristics of the insulator or circulator cannot be achieved.